A clinical and scientific goal for many years has been to determine cardiac output (i.e. blood flow ejected from the heart) from an arterial pressure measurement. The arterial pressure and flow pulse waveform is created by this ejection, but on account of: differences in arterial properties with age; differences in the pattern of flow ejection to the heart with age; weakening of the left ventricular muscle; and change in the heart rate under resting conditions, this goal has not yet been obtained with precision. An accurate determination of cardiac output is useful for diagnosis of cardiovascular diseases and clinically evaluating cardiac function. Cardiac output is also used as a guide in monitoring the therapeutic effects of a treatment in clinical situations.
Many methods are known for estimating cardiac output, either invasively or non-invasively, as there is no way to directly measure cardiac output. The Swan-Ganz thermo-dilution and the Dilution methods using the Stewart-Hamilton equation are the most used and accurate in measuring cardiac output. However, a disadvantage of these methods is that they are invasive procedures, which limits their application.
A non-invasive method uses Fick's principle to correlate the consumption of Oxygen, measured by a spirometer, with cardiac output. However, a disadvantage of this method is that it is time consuming due to the difficulty of collecting and analysing the gas concentrations.
Another non-invasive method requires echocardiography using an ultrasound Doppler probe to measure flow waves with echocardiographic measurements of aortic cross sectional area to calculate cardiac output (GE Healthcare, USA; Philips, The Netherlands). However, disadvantages of this method include that: its accuracy is dependant on the skill and accuracy of the probe location; and it is dependant on other measurements beside the flow waveform.
The PiCCO (PULSION Medical Systems AG, Munich, Germany) and PulseCO (LiDCO Ltd, London, England) technologies analyse arterial pressure waveforms to estimate cardiac output. These methods can estimate continuous cardiac output. However, a disadvantage of these methods is that they require an invasive calibration using either transpulmonary thermodilution or lithium dilution techniques, which diminishes their suitability for use as clinical devices.
The FloTrac method (Edward Lifesciences, USA) does not require independent calibration and estimates cardiac output and stroke volume from an arterial pressure waveform only. Disadvantages of this method include that: the calculation is based on a statistical model that correlates beat to beat stroke volume with beat to beat pulse pressure, age, gender and body surface area and the accuracy of the method is dependant on the accuracy of measurements used in calculating the model and on the size and representativeness of the population used to establish the model; and this technology requires an invasive arterial pressure waveform, which limits its suitability for a clinical application.
Another non-invasive method for estimating cardiac output is the thoracic bio-impedance method, which measures changing impedance in the chest as the heart beat changes fluid volumes and from this estimates cardiac output. Examples of such products are Bio-Z (Cardiodynamics Inc, USA) and PhysioFlow (Manatec Biomedical, France). Disadvantages of this method include that it: is expensive, both in terms of capital equipment cost and the consumables used per test; takes a relatively long time to set up sensors and stabilise readings; and still requires more than one signal (ECG, SpO2, and phonocardiogram) to calculate cardiac output.
Another non-invasive method for estimating cardiac output is the use of Doppler ultrasound to measure aortic blood flow (e.g. the USCOM device, USCOM Limited, Australia). Disadvantages of this method include that it: requires a carefully located and maintained Doppler sensor “looking” through the suprastemal notch at blood flow in the ascending aorta; requires operator skill not commonly available in a primary care doctor's office setting; and is expensive.
International PCT Patent Application No. PCT/AU2005/000311 (published as WO 2005/084536) discloses a method and apparatus for determining cardiac output from a peripheral pressure waveform. However, the disclosed method, which is broken down into 8 basic steps, suffers from the following disadvantages.
In relation to step 3, the disclosed method relies on a linear equation to calculate aortic Pulse Wave Velocity (PWV) from Age. The PWV equation was not mentioned in the reference cited, but estimated indirectly. Therefore, this equation is of questionable accuracy. Furthermore, in regard to the PWV measurement used to calculate the equation, there are only two types of aortic PWV measurements: invasive; and non invasive. If PWV was measured invasively, then (due to the necessary invasive procedures) the equation had to be derived from people suffering from cardiovascular disease. The disadvantage of such an invasive approach is the equation is thus not representative of a healthy population. If PWV was measured non-invasively, then the equation had to use a surrogate measurement of aortic PWV, which means that the equation does not relate aortic PWV with age but other PWV measurements, which will increase the error in the ensuing calculation of peak flow that uses aortic PWV. Also, the PWV equation assumes that PWV is linear with age. However, this is also questionable as other papers show a nonlinear relationship between age and aortic PWV.
In relation to step 4, the equation used to adjust aortic PWV to mean pressure was not mentioned in the associated reference, which casts doubt on the equation's accuracy.
In relation to steps 6-8, the equations used to adjust the flow for age and heart rate are not referenced. The accuracy of these equations is thus questionable. Further, the reason for all these adjustments is the unavailability of flow wave (either estimated or measured). This will make all of the flow adjustment equations less accurate.
By way of further background, the disclosure of the Applicant's International PCT Patent Application no. PCT/AU2006/001789, now U.S. Pat. No. 8,273,030, entitled: A method of estimating Pulse Wave Velocity (hereafter referred to as “the PWV application”) is incorporated herein by cross reference. The PWV application discloses a method of calculating aortic PWV from central pressure waveform (CPW) by decomposing CPW into forward and reflected waveforms (referred to herein as the “AVI calculation”).